Miniaturized plasma sources are used in a variety of applications, e.g., in chemical analysis, for sterilization or activation of different substances, or as ion sources. Some of their advantages are low power consumption, simple design and fabrication, mechanical robustness, long lifetime, high non-thermal plasma density, and the ability to operate at atmospheric pressure, although they also suffer from some limitations.
Miniaturized plasma sources based on microstrip split-ring resonators (MSRRs) have received an increasing amount of interest during the last decade. A microstrip is a radio frequency transmission line consisting of a dielectric substrate sandwiched between a metal strip and a metal ground plane, as shown in FIG. 1(a). In the case of an MSRR, the metal strip is a dipole folded into a circle, with the two ends creating a small gap, as shown in FIG. 1(b), over which an electric field can be created. Here, the dipole corresponds to a half-wavelength microwave resonator often used as an antenna. The dipole is connected to an additional microstrip transmission line, a feed line that supplies it with radio frequency (RF) power.
At resonance, the electric potential at the ends of the dipole is 180° out of phase, enabling the amplitude of the electric field between them to be amplified several orders of magnitude. Hence, with a relatively low input power, a large potential is created over the gap, and this potential is used to ignite and maintain a plasma, as shown in FIG. 1(c). The large potential arises because when a standing wave is created in a dipole, the current is zero at the endpoints and at its maximum at the middle of the strip. Correspondingly, the electric potential is at its maximum and minimum, respectively, at the endpoints, although the two ends are 180° out of phase, and zero at the middle.
The electric field in a microstrip is mostly confined to the dielectric substrate. However, in the gap of a split-ring resonator, the electric field between the ends of the folded dipole is elevated from the substrate and is concentrated in the plane between the two ends. This provides one of the key features of the MSRR, i.e., that most of the electric field is concentrated to the gap. It should be noted that no DC potential is applied to the plasma, minimizing energy lost to moving ions.
In order to effectively operate the MSRR, the input impedance of the ring should match the impedance of the feed line. The characteristic impedance of the ring depends on the offset of the feed line from the center of the dipole, and the quality factor of the microstrip.
One laser spectroscopy technique using a plasma source is optogalvanic spectroscopy (OGS) in general and intracavity optogalvanic spectroscopy (ICOGS) in particular. Both are based on the optogalvanic (OG) effect. Using this, the interaction of an incident laser beam with atoms or molecules present in a plasma induces changes in the electrical properties (e.g., voltage or impedance) of the plasma which can be measured electrically. The measured impedance change is proportional to the number of interacting molecules but will also depend on the plasma parameters, e.g., pressure, as well as the laser intensity and gas composition in the discharge.
By using an isotope-specific CO2 laser as the source of the radiation, i.e. a laser with a wavelength identical to one of the isotope-specific transitions in the mid-IR spectrum of CO2, it has been shown that OGS can be applicable to measurements of the isotopic composition of carbon-containing samples, e.g., the 13C/12C ratio, for, e.g., ulcer diagnostics, and that it might even be possible to measure the 14C/12C ratio using the ICOGS technique. The schematic of a standard ICOGS system is shown in FIG. 2. As shown in FIG. 2, a sample cell 10 is inserted into the laser cavity 14 of a laser 15, with a lasing frequency corresponding to a transition in the spectrum of the active laser medium 16 (which may be a moiety of interest, e.g, a gas mixture for a CO2 laser) which receives a potential from power source 13′, which may be different from the power source 13 which drives a plasma 6 in the sample cell 10. A sample can be inserted into the sample cell 10, via a fluidic system 9, from an analyte source 17. The pressure in, or the flow through, the sample cell 10 is defined by a vacuum pump 18. The sample cell 10 and the active laser medium 16 are located between two reflectors 22 and 22′ in the cavity 14 of the laser 15, where the first reflector 22 can be an output coupler and the second reflector 22′ can be a grating. A shutter 19 connected to a modulator 20 chops the laser beam 7, creating a frequency dependent optogalvanic response in the sample cell 10, which is measured by a detector 12, connected to the power source 13. The measured signal is demodulated in a signal processing unit 21 triggered by the modulator 20, yielding the optogalvanic signal. For quantitative measurements, the sample cell 10 is illuminated by a second laser beam from a reference laser 15′. The reference laser 15′ has a separate shutter 19′, operating at a different frequency than the shutter 19 of the laser 15 with the laser medium 16. Two optogalvanic responses are measured by the detector 12, and demodulated by the signal processing unit 21 into two unambiguous optogalvanic signals.
In ordinary OGS, the ionized sample is located in the laser beam path, i.e. outside the laser, whereas in ICOGS the sample is inserted into the laser cavity itself. In both cases, the analyzed carbon is in the form of CO2. The intracavity approach has been suggested to increase the sensitivity by almost seven orders of magnitude compared with ordinary “extracavity” OGS. However, in many applications, the sensitivity of extracavity OGS is sufficient, and the plasma source can be kept outside the laser. A spectrometer for extracavity OGS of 13C/12C ratios is sometimes referred to as a laser-assisted ratio analyzer (LARA).